Sensors using an evanescent field have become extremely useful in the past few years. These sensors have been created to detect a wide variety of materials such as pesticides, pathogens, nucleic acids, gases, and disease markers such as the HER2 breast cancer marker. These sensors function by measuring the interaction of an evanescent field (a non-propagating near field optical wave) with a target.
Total Internal Reflection Photoacoustic Spectroscopy (TIRPAS) is a method that exploits the evanescent field of a laser pulse reflecting off a glass/sample interface to generate photoacoustic responses. Specifically, the photoacoustic responses are typically generated by light absorbing analytes in a fluid sample (typically a liquid) that is in contact with a prism in the event one or more of the analytes are within the penetration depth of the evanescent field because upon absorbing the light energy the temperature of the analyte rapidly increases causing a rapid expansion (typically thermo-elastic) that in turns results in the formation of acoustic waves that propagate through the sample to a sensor.
For example, TIRPAS has been employed to detect dyes in a sample. Hinoue et al., Photoacoustic Observation of Solid-liquid Interface by Means of Total Internal Reflection Technique, CHEMISTRY LETTERS, 225-228 (1983). Hinoue et al. used a laser pulse generated with an optically chopped continuous beam HeNe laser at 632.8 nm to detect Brilliant Blue FCF dye at different angles of incidence to generate the evanescent field with a lock-in amplifier, which only amplifies a specific frequency, to detect the resulting acoustic wave. Although TIRPAS is typically used to analyze liquid samples, it may be used on gaseous and solid samples. See, e.g., Muessig et al., Total Internal Reflectance Optoacoustic Spectroscopy, J. APPL. PHYS. 54(8), 4251-4253 (1983). Muessig et al. also used continuous laser irradiation but instead of using an optical chopper to produce laser pulses, Muessig et al. used an oscillator connected to a lock-in amplifier to produce laser pulses.
Although TIRPAS has been known for more than thirty years its use has been limited due to shortcomings. For example, the TIRPAS disclosed by Hinoue et al. was limited to detecting Brilliant Blue FCF dye, a relatively high absorption analyte. Thus, it is unable to provide meaningful detection of low absorption analytes such as those that may be present in biological samples. In view of the foregoing, a need still exists for system(s) and method(s) for conducting TIRPAS that reduces or eliminates one or more of the foregoing shortcomings.